We need a toll producer to manufacture product X. Where do we find one that understands the product requirement and is willing to scale up production if demand increases?

There are a number of toll producers around the world. These can be shortlisted, depending on the specific requirements for producing product X (quantity, raw materials and process, for example). Subsequently, check their (i) availability, (ii) flexibility in meeting your uncertain demands and (iii) willingness to conduct trials and share risk.